1. Field of the Invention
The present invention relates to an automatic power control (APC) circuit for controlling the transmitting power of a modulated radio frequency (RF) signal and particularly to such a circuit suitable for use in a time division multiple access (TDMA) system in which the RF signal is required to be linearly amplified.
2. Description of the Related Art
In the accompanying drawings, FIG. 1 shows a conventional power amplifying device having an APC function. After a modulated RF signal 1 has been regulated in level by a variable gain amplifier 2, it is linearly amplified to a predetermined transmitting power by a power amplifier 3, the amplified signal being then sent out as an RF output signal 4. The modulated RF signal 1 is one that has envelope fluctuations such as a .pi./4 shift QPSK signal. .pi./4 shift QPSK has one set of 4 signal points (0, .pi./2, .pi., 3.pi./2) and one other set of signal points (.pi./4, 3.pi./4, 5.pi./4, 7.pi./4) which are shifted by .pi./4 from the first set of signal points, and these two sets of signal points are used alternately for every two bits. Therefore, every time the signal point changes, the carrier wave changes discontinuously and the amplitude modulation component is generated. This amplitude modulation component causes fluctuation of the envelope of the RF signal.
Even if the level of the inputted RF signal 1 happens to fluctuate or even if there are fluctuations in supply voltage, the level of the RF output signal 4 must be within a predetermined range. In such a relatively small-sized wireless transmitter-receiver as is normal for an in-vehicle type or portable type, however, the supply voltage tends to fluctuate, resulting in fluctuations of the RF output signal level. It is therefore required to monitor the level of the RF output signal 4 at all times such that the variable gain amplifier 2 can be subjected to feedback to suppress the fluctuations in the RF output signal 4. To this end, a detector 5 detects the RF output signal 4 to provide a detection signal 6. This detection signal 6 represents the envelope of the RF output signal 4.
The detection signal 6, that is the envelope of the RF output signal 4, is provided to a fluctuation removing circuit 8 wherein the detection signal 6 is compared with an envelope 7 which is obtained from a base band signal. The envelope 7 has already been calculated by an envelope calculating unit 13 as a square root of (I.sup.2 +Q.sup.2), where I is in-phase component of the base band signal and Q is orthogonal component of the base band signal.
The fluctuation removing circuit 8 outputs a difference between the detection signal 6 and the envelope 7 calculated from the base band signal. Such a difference between two envelopes does not contain fluctuation components which are produced by the modulation.
The output of the fluctuation removing circuit 8 is inputted into a comparator 9 which in turn outputs a difference signal between the output signal of the fluctuation removing circuit 8 and a reference voltage 10 outputted from a reference voltage generator 14 and corresponding to the transmitting power. The difference signal is passed through a low-pass filter 11 and outputted as a feedback control signal 12 after any influence of noise or the like has been removed. The feedback control signal 12 is fed back to the gain regulating terminal 2a of the variable gain amplifier 2 which is located forward of the power amplifier 3.
In such a manner, the transmitting power of the RF output signal 4 in the same power level can be controlled by the variable gain amplifier 2 which is disposed on the input side of the power amplifier 3. More particularly, the feedback control signal 12 functions to reduce the gain of the variable gain amplifier 2 if the transmitting power of the RF signal increases. When the transmitting power of the RF signal decreases, the feedback control signal 12 functions to increase the gain of the variable gain amplifier 2. Thus, the RF output signal 4 can have a constant average power.
In such an arrangement, the APC circuit of the prior art must determine an envelope from the computation of a base band signal to compare it with the detection signal of the RF output signal. The APC circuit of the related art raises a problem in that it requires complicated and large-scaled hardware for calculating the envelope from the base band signal.